1. Field of the Invention
This invention relates to a device and method for use in an ion chromatographic system for the analysis of trace levels of perchlorate. More particularly, the device and method concentrates perchlorate and reduces the level of interference from other matrix bound components in a test specimen, thereby enhancing the detection of the perchlorate within the test specimen.
2. Description of the Related Art
Perchlorate is a chemical that exists in the environment as a part of other compounds, such as ammonium, potassium, or sodium perchlorate. Ammonium perchlorate, which comprises the bulk of manufactured perchlorate, is used as an oxygen-adding component in solid fuel propellant for rockets, missiles, and fireworks. Because of its limited shelf-life, inventories of ammonium perchlorate must be periodically replaced. Thus, large volumes of the compound have been disposed of since the 1950""s.
Until recently, perchlorate containing compounds were not suspected as being detrimental to health. In fact, potassium perchlorate was used therapeutically to treat hyperthyroidism. However, recent studies have shown that perchlorate can affect the thyroid gland, and, therefore, affect metabolism, growth, and development. Due to these studies, the federal Environmental Protection Agency (EPA) has put perchlorate on its Contaminant Candidate List for further study and potential regulatory action. Both California and Nevada have set action levels of eighteen parts per billion for perchlorate under their drinking water regulations. In a report published in January of 2002, the EPA have set a proposed action limit for perchlorate at 1.5 parts per billion. Because current regulatory actions regarding perchlorate have begun and future regulatory actions regarding perchlorate appear certain, regulatory agencies have focused upon testing methods for perchlorate.
The only current method of detecting perchlorate in samples found by the regulatory agencies is using ion chromatography. This method of detection applies two basic components: (1) separation of perchlorate from all other species in a sample, and (2) measurement of the separated perchlorate against suitable standards. Separation of perchlorate in a sample is based upon the attraction of perchlorate for a special organic exchanger packed into a column. The anionics are carried through the column by a flow of solution. As the anionics move through the column, they separate into thin bands. Because the relative strength of the attraction of the different anionics to the ion exchanger is different for each of the species dissolved in the sample, they separate and elute from the ion exchange column at different times. As the anionics pass through the detector, the detector response is registered as peaks with a peak amplitude proportional to concentration and at a retention time characteristic of the anionic. This is accomplished through a specific conductance detector that measures the conductance relative to the background which is generated by the mobile phase carrying the analyte through the system.
The hardware used for this type of analysis is standard equipment used in ion chromatographic systems. The current federal EPA published method for detecting perchlorate calls for an ion chromatographic system comprised of an ion chromatographic pump, sample injection valve, guard column, analytical column, suppressor device, and conductivity detector. However, there are problems associated with obtaining low levels of perchlorate in certain types of samples using this standard ion chromatography configuration. Interferences caused by a large amount of anionics other than perchlorate within a sample can lead to false positives and/or reduced detection limits. The federal EPA method suggests that pretreating the sample through dilution can potentially assist with these problems, but the dilution may cause a reduction of the concentration of the target analyte to the point where it becomes undetectable. These problems are especially problematic in samples obtained from sources that contain extremely complex matrices of components, such as seawater. In practice, this current detection method is capable of relatively low detection levels of perchlorate in samples with low levels of ionic interferences. However, prior to the present invention, no known analysis method or device can meet the proposed action limit being considered by the EPA mentioned above.
The present invention solves the problems noted above related to the standard ion chromatographic method of perchlorate detection. The present invention comprises using a concentrator column, optimized to remove all of the perchlorate within a sample, in order to dramatically increase the overall effectiveness of detection of perchlorate within an ion chromatograph system, while reducing the waste produced by the system, and providing significantly shorter analysis times. By using the present invention, the sensitivity of perchlorate detection is increased by one hundred to one thousand times that of the current detection method described above.
Accordingly, an object of this invention is to increase the sensitivity of perchlorate detection in liquid samples.
A further object of the invention is to allow detection of low amounts of perchlorate in samples with complex matrices of ions.
This invention accomplishes these objectives and other needs related to perchlorate detection in environmental matrices by providing a device used within an ion chromatograph system to analyze the content of perchlorate within a sample. The device consists of a concentrator column that is optimized to remove all of the perchlorate within a sample while providing for minimum waste produced by the system. This is due to using a packing material within the concentrator column that has a high affinity for perchlorate ions, but a lower affinity for other high ionic organic or inorganic interferants. The concentrator column accomplishes these tasks by using a packing material comprising an anionic capacity of from about 0.030 to about 0.060 microequivalents per column, with a preferred anionic capacity of from about 0.035 to about 0.040 microequivalents per column, and a most preferred anionic capacity of about 0.036 microequivalents per column. Although applicant is unaware of any commercial packing material having such a capacity, this invention also includes such a packing material produced by blending currently available packing materials, having anionic capacities that are higher and lower than these capacities. The improved packing material is made from blending a material comprising from about twenty-five percent to about eighty percent of a low anionic capacity material comprising an anionic capacity of less than about 0.03 milliequivalents per milliliter mixed with another material comprising from about twenty-five percent to about eighty percent of a high anionic capacity material comprising an anionic capacity of more than about 0.07 milliequivalents per milliliter. The invention also includes using the concentrator column device described above within an ion chromatographic system in order to detect trace levels of perchlorate in water samples.